F-actin waves, actin cortex disassembly and focal exocytosis driven by actin-phosphoinositide positive feedback

PIP5K-Ras bistability initiates plasma membrane symmetry breaking to regulate cell polarity and migration

Symmetry breaking, polarity establishment, and spontaneous cell protrusion formation are fundamental but poorly explained cell behaviors. Here, we demonstrate that a biochemical network, where the mutually inhibitory localization of PIP5K and Ras activities plays a central role, governs these processes. First, in resting cells devoid of cytoskeletal activity, PIP5K is uniformly elevated on the plasma membrane, while Ras activity remains minimal. Symmetry is broken by spontaneous local displacements of PIP5K, coupled with simultaneous activations of Ras and downstream signaling events, including PI3K activation. Second, knockout of PIP5K dramatically increases both the incidence and size of Ras-PI3K activation patches, accompanied by branched F-actin assembly. This leads to enhanced cortical wave formation, increased protrusive activity, and a shift in migration mode. Third, high inducible overexpression of PIP5K virtually eliminates Ras-PI3K signaling, cytoskeletal activity, and cell migration, while acute recruitment of cytosolic PIP5K to the membrane induces contraction and blebs in cancer cells. These arrested phenotypes are reversed by reducing myosin II activity, indicating myosin's involvement in the PIP5K-Ras-centered regulatory network. Remarkably, low inducible overexpression of PIP5K unexpectedly facilitates polarity establishment, highlighting PIP5K as a highly sensitive master regulator of these processes. Simulations of a computational model combining an excitable system, cytoskeletal loops, and dynamic partitioning of PIP5K recreates the experimental observations. Taken together, our results reveal that a bistable, mutually exclusive localization of PIP5K and active Ras on the plasma membrane triggers the initial symmetry breaking. Coupled actomyosin reduction and increased actin polymerization lead to intermittently extended protrusions and, with feedback from the cytoskeleton, self-organizing, complementary gradients of PIP5K versus Ras steepen, raising the threshold of the networks at the rear and lowering it at the front to generate polarity for cell migration.

Competition and synergy of Arp2/3 and formins in nucleating actin waves

Actin assembly and dynamics are crucial for maintaining cell structure and changing physiological states. The broad impact of actin on various cellular processes makes it challenging to dissect the specific role of actin regulatory proteins. Using actin waves that propagate on the cortex of mast cells as a model, we discovered that formins (FMNL1 and mDia3) are recruited before the Arp2/3 complex in actin waves. GTPase Cdc42 interactions drive FMNL1 oscillations, with active Cdc42 and the constitutively active mutant of FMNL1 capable of forming waves on the plasma membrane independently of actin waves. Additionally, the delayed recruitment of Arp2/3 antagonizes FMNL1 and active Cdc42. This antagonism is not due to competition for monomeric actin but rather for their common upstream regulator, active Cdc42, whose levels are negatively regulated by Arp2/3 via SHIP1 recruitment. Collectively, our study highlights the complex feedback loops in the dynamic control of the actin cytoskeletal network.

Mitochondrially-associated actin waves maintain organelle homeostasis and equitable inheritance

First identified in dividing cells as revolving clusters of actin filaments, these are now understood as mitochondrially-associated actin waves that are active throughout the cell cycle. These waves are formed from the polymerization of actin onto a subset of mitochondria. Within minutes, this F-actin depolymerizes while newly formed actin filaments assemble onto neighboring mitochondria. In interphase, actin waves locally fragment the mitochondrial network, enhancing mitochondrial content mixing to maintain organelle homeostasis. In dividing cells actin waves spatially mix mitochondria in the mother cell to ensure equitable partitioning of these organelles between daughter cells. Progress has been made in understanding the consequences of actin cycling as well as the underlying molecular mechanisms, but many questions remain, and here we review these elements. Also, we draw parallels between mitochondrially-associated actin cycling and cortical actin waves. These dynamic systems highlight the remarkable plasticity of the actin cytoskeleton.

Nonlinear dynamics in phosphoinositide metabolism

Phosphoinositides broadly impact membrane dynamics, signal transduction and cellular physiology. The orchestration of signaling complexity by this seemingly simple metabolic pathway remains an open question. It is increasingly evident that comprehending the complexity of the phosphoinositides metabolic network requires a systems view based on nonlinear dynamics, where the products of metabolism can either positively or negatively modulate enzymatic function. These feedback and feedforward loops may be paradoxical, leading to counterintuitive effects. In this review, we introduce the framework of nonlinear dynamics, emphasizing distinct dynamical regimes such as the excitable state, oscillations, and mixed-mode oscillations-all of which have been experimentally observed in phosphoinositide metabolisms. We delve into how these dynamical behaviors arise from one or multiple network motifs, including positive and negative feedback loops, coherent and incoherent feedforward loops. We explore the current understanding of the molecular circuits responsible for these behaviors. While mapping these circuits presents both conceptual and experimental challenges, redefining cellular behavior based on dynamical state, lipid fluxes, time delay, and network topology is likely essential for a comprehensive understanding of this fundamental metabolic network.

A dynamic partitioning mechanism polarizes membrane protein distribution

The plasma membrane is widely regarded as the hub of the numerous signal transduction activities. Yet, the fundamental biophysical mechanisms that spatiotemporally compartmentalize different classes of membrane proteins remain unclear. Using multimodal live-cell imaging, here we first show that several lipid-anchored membrane proteins are consistently depleted from the membrane regions where the Ras/PI3K/Akt/F-actin network is activated. The dynamic polarization of these proteins does not depend upon the F-actin-based cytoskeletal structures, recurring shuttling between membrane and cytosol, or directed vesicular trafficking. Photoconversion microscopy and single-molecule measurements demonstrate that these lipid-anchored molecules have substantially dissimilar diffusion profiles in different regions of the membrane which enable their selective segregation. When these diffusion coefficients are incorporated into an excitable network-based stochastic reaction-diffusion model, simulations reveal that the altered affinity mediated selective partitioning is sufficient to drive familiar propagating wave patterns. Furthermore, normally uniform integral and lipid-anchored membrane proteins partition successfully when membrane domain-specific peptides are optogenetically recruited to them. We propose "dynamic partitioning" as a new mechanism that can account for large-scale compartmentalization of a wide array of lipid-anchored and integral membrane proteins during various physiological processes where membrane polarizes.

Competition and Synergy of Arp2/3 and Formins in Nucleating Actin Waves

The assembly and disassembly of actin filaments and their regulatory proteins are crucial for maintaining cell structure or changing physiological state. However, because of the tremendous global impact of actin on diverse cellular processes, dissecting the specific role of actin regulatory proteins remains challenging. In this study, we employ actin waves that propagate on the cortex of mast cell to investigate the interplay between formins and the Arp2/3 complex in the nucleating and turnover of cortical actin. Our findings reveal that the recruitment of FMNL1 and mDia3 precedes the Arp2/3 complex in cortical actin waves. Membrane and GTPase-interaction can drive oscillations of FMNL1 in an actin-dependent manner, but active Cdc42 waves or constitutively-active FMNL1 mutant can form without actin waves. In addition to the apparent coordinated assembly of formins and Arp2/3, we further reveal their antagonism, where inhibition of Arp2/3 complex by CK-666 led to a transient increase in the recruitment of formins and actin polymerization. Our analysis suggest that the antagonism could not be explained for the competition between FMNL1 and Arp2/3 for monomeric actin. Rather, it is regulated by a limited pool of their common upstream regulator, Cdc42, whose level is negatively regulated by Arp2/3. Collectively, our study highlights the multifaceted interactions, cooperative or competitive, between formins and Arp2/3 complex, in the intricate and dynamic control of actin cytoskeletal network.

Periodicity, mixed-mode oscillations, and multiple timescales in a phosphoinositide-Rho GTPase network

While rhythmic contractile behavior is commonly observed at the cellular cortex, the primary focus has been on excitable or periodic events described by simple activator-delayed inhibitor mechanisms. We show that Rho GTPase activation in nocodazole-treated mitotic cells exhibits both simple oscillations and complex mixed-mode oscillations. Rho oscillations with a 20- to 30-s period are regulated by phosphatidylinositol (3,4,5)-trisphosphate (PIP3) via an activator-delayed inhibitor mechanism, while a slow reaction with period of minutes is regulated by phosphatidylinositol 4-kinase via an activator-substrate depletion mechanism. Conversion from simple to complex oscillations can be induced by modulating PIP3 metabolism or altering membrane contact site protein E-Syt1. PTEN depletion results in a period-doubling intermediate, which, like mixed-mode oscillations, is an intermediate state toward chaos. In sum, this system operates at the edge of chaos. Small changes in phosphoinositide metabolism can confer cells with the flexibility to rapidly enter ordered states with different periodicities.

From actin waves to mechanism and back: How theory aids biological understanding

Actin dynamics in cell motility, division, and phagocytosis is regulated by complex factors with multiple feedback loops, often leading to emergent dynamic patterns in the form of propagating waves of actin polymerization activity that are poorly understood. Many in the actin wave community have attempted to discern the underlying mechanisms using experiments and/or mathematical models and theory. Here, we survey methods and hypotheses for actin waves based on signaling networks, mechano-chemical effects, and transport characteristics, with examples drawn from Dictyostelium discoideum, human neutrophils, Caenorhabditis elegans, and Xenopus laevis oocytes. While experimentalists focus on the details of molecular components, theorists pose a central question of universality: Are there generic, model-independent, underlying principles, or just boundless cell-specific details? We argue that mathematical methods are equally important for understanding the emergence, evolution, and persistence of actin waves and conclude with a few challenges for future studies.

Spatiotemporal dynamics of membrane surface charge regulates cell polarity and migration

During cell migration and polarization, numerous signal transduction and cytoskeletal components self-organize to generate localized protrusions. Although biochemical and genetic analyses have delineated many specific interactions, how the activation and localization of so many different molecules are spatiotemporally orchestrated at the subcellular level has remained unclear. Here we show that the regulation of negative surface charge on the inner leaflet of the plasma membrane plays an integrative role in the molecular interactions. Surface charge, or zeta potential, is transiently lowered at new protrusions and within cortical waves of Ras/PI3K/TORC2/F-actin network activation. Rapid alterations of inner leaflet anionic phospholipids-such as PI(4,5)P2, PI(3,4)P2, phosphatidylserine and phosphatidic acid-collectively contribute to the surface charge changes. Abruptly reducing the surface charge by recruiting positively charged optogenetic actuators was sufficient to trigger the entire biochemical network, initiate de novo protrusions and abrogate pre-existing polarity. These effects were blocked by genetic or pharmacological inhibition of key signalling components such as AKT and PI3K/TORC2. Conversely, increasing the negative surface charge deactivated the network and locally suppressed chemoattractant-induced protrusions or subverted EGF-induced ERK activation. Computational simulations involving excitable biochemical networks demonstrated that slight changes in feedback loops, induced by recruitment of the charged actuators, could lead to outsized effects on system activation. We propose that key signalling network components act on, and are in turn acted upon, by surface charge, closing feedback loops, which bring about the global-scale molecular self-organization required for spontaneous protrusion formation, cell migration and polarity establishment.

Versatile Patterns in the Actin Cortex of Motile Cells: Self-Organized Pulses Can Coexist with Macropinocytic Ring-Shaped Waves

Self-organized patterns in the actin cytoskeleton are essential for eukaryotic cellular life. They are the building blocks of many functional structures that often operate simultaneously to facilitate, for example, nutrient uptake and movement of cells. However, identifying how qualitatively distinct actin patterns can coexist remains a challenge. Using bifurcation theory of a mass conserved activator-inhibitor system, we uncover a generic mechanism of how different actin waves-traveling waves and excitable pulses-organize and simultaneously emerge. Live-cell imaging experiments indeed reveal that narrow, planar, and fast-moving excitable pulses may coexist with ring-shaped macropinocytic actin waves in the cortex of motile amoeboid cells.

Building the phagocytic cup on an actin scaffold

Cells ingest large particles, such as bacteria, viruses, or apoptotic cells, via the process of phagocytosis, which involves formation of an actin-rich structure known as the phagocytic cup. Phagocytic cup assembly and closure results from a concerted action of phagocytic receptors, regulators of actin polymerization, and myosin motors. Recent studies using advanced imaging approaches and biophysical techniques have revealed new information regarding phagocytic cup architecture, regulation of actin assembly, and the distribution, direction, and magnitude of the forces produced by the cytoskeletal elements that form the cup. These findings provide insights into the mechanisms leading to the assembly, expansion, and closure of phagocytic cups. The new data show that engulfment and internalization of phagocytic targets rely on several distinct yet complementary mechanisms that support the robust uptake of foreign objects and may be precisely tailored to the demands of specific phagocytic pathways.

OSCILLATORS IN THE MICROVASCULATURE - GLYCOCALYX AND BEYOND

The growing recognition of abundance of oscillating functions in biological systems has motivated this brief overview which narrows down on the microvasculature. Specifically, it encompasses self-sustained oscillations of blood flow, hematocrit and viscosity at bifurcations; their effects on the oscillations of endothelial glycocalyx, mechanotransduction and its termination to prime endothelial cells for the subsequent mechanical signaling event; oscillating affinity of hyaluronan-CD44 binding domain; spontaneous contractility of actomyosin complexes in the cortical actin web, its effects on the tension of the plasma membrane; reversible effects of sirtuin-1 on endothelial glycocalyx; and effects of plasma membrane tension on endo-and exocytosis. Some potential interactions between those oscillators - their coupling - are discussed together with their transition into chaotic movements. Future in-depth understanding of the oscillatory activities in the microvasculature could serve as a guide to its chronotherapy under pathological conditions.

Microtopographical guidance of macropinocytic signaling patches

Morphologies of amoebae and immune cells are highly deformable and dynamic, which facilitates migration in various terrains, as well as ingestion of extracellular solutes and particles. It remains largely unexplored whether and how the underlying membrane protrusions are triggered and guided by the geometry of the surface in contact. In this study, we show that in Dictyostelium, the precursor of a structure called macropinocytic cup, which has been thought to be a constitutive process for the uptake of extracellular fluid, is triggered by micrometer-scale surface features. Imaging analysis and computational simulations demonstrate how the topographical dependence of the self-organizing dynamics supports efficient guidance and capturing of the membrane protrusion and hence movement of an entire cell along such surface features.

In fast-moving cells such as amoeba and immune cells, dendritic actin filaments are spatiotemporally regulated to shape large-scale plasma membrane protrusions. Despite their importance in migration, as well as in particle and liquid ingestion, how their dynamics are affected by micrometer-scale features of the contact surface is still poorly understood. Here, through quantitative image analysis of Dictyostelium on microfabricated surfaces, we show that there is a distinct mode of topographical guidance directed by the macropinocytic membrane cup. Unlike other topographical guidance known to date that depends on nanometer-scale curvature sensing protein or stress fibers, the macropinocytic membrane cup is driven by the Ras/PI3K/F-actin signaling patch and its dependency on the micrometer-scale topographical features, namely PI3K/F-actin–independent accumulation of Ras-GTP at the convex curved surface, PI3K-dependent patch propagation along the convex edge, and its actomyosin-dependent constriction at the concave edge. Mathematical model simulations demonstrate that the topographically dependent initiation, in combination with the mutually defining patch patterning and the membrane deformation, gives rise to the topographical guidance. Our results suggest that the macropinocytic cup is a self-enclosing structure that can support liquid ingestion by default; however, in the presence of structured surfaces, it is directed to faithfully trace bent and bifurcating ridges for particle ingestion and cell guidance.

A phosphoinositide-based model of actin waves in frustrated phagocytosis

Phagocytosis is a complex process by which phagocytes such as lymphocytes or macrophages engulf and destroy foreign bodies called pathogens in a tissue. The process is triggered by the detection of antibodies that trigger signaling mechanisms that control the changes of the cellular cytoskeleton needed for engulfment of the pathogen. A mathematical model of the entire process would be extremely complicated, because the signaling and cytoskeletal changes produce large mechanical deformations of the cell. Recent experiments have used a confinement technique that leads to a process called frustrated phagocytosis, in which the membrane does not deform, but rather, signaling triggers actin waves that propagate along the boundary of the cell. This eliminates the large-scale deformations and facilitates modeling of the wave dynamics. Herein we develop a model of the actin dynamics observed in frustrated phagocytosis and show that it can replicate the experimental observations. We identify the key components that control the actin waves and make a number of experimentally-testable predictions. In particular, we predict that diffusion coefficients of membrane-bound species must be larger behind the wavefront to replicate the internal structure of the waves. Our model is a first step toward a more complete model of phagocytosis, and provides insights into circular dorsal ruffles as well.

Rapid viscoelastic changes are a hallmark of early leukocyte activation

To accomplish their critical task of removing infected cells and fighting pathogens, leukocytes activate by forming specialized interfaces with other cells. The physics of this key immunological process are poorly understood, but it is important to understand them because leukocytes have been shown to react to their mechanical environment. Using an innovative micropipette rheometer, we show in three different types of leukocytes that, when stimulated by microbeads mimicking target cells, leukocytes become up to 10 times stiffer and more viscous. These mechanical changes start within seconds after contact and evolve rapidly over minutes. Remarkably, leukocyte elastic and viscous properties evolve in parallel, preserving a well-defined ratio that constitutes a mechanical signature specific to each cell type. Our results indicate that simultaneously tracking both elastic and viscous properties during an active cell process provides a new, to our knowledge, way to investigate cell mechanical processes. Our findings also suggest that dynamic immunomechanical measurements can help discriminate between leukocyte subtypes during activation.

Imaging methods in mechanosensing: a historical perspective and visions for the future

Over the past three decades, as mechanobiology has become a distinct area of study, researchers have developed novel imaging tools to discover the pathways of biomechanical signaling. Early work with substrate engineering and particle tracking demonstrated the importance of cell–extracellular matrix interactions on the cell cycle as well as the mechanical flux of the intracellular environment. Most recently, tension sensor approaches allowed directly measuring tension in cell–cell and cell–substrate interactions. We retrospectively analyze how these various optical techniques progressed the field and suggest our vision forward for a unified theory of cell mechanics, mapping cellular mechanosensing, and novel biomedical applications for mechanobiology.

Complementary mesoscale dynamics of spectrin and acto-myosin shape membrane territories during mechanoresponse

The spectrin-based membrane skeleton is a major component of the cell cortex. While expressed by all metazoans, its dynamic interactions with the other cortex components, including the plasma membrane or the acto-myosin cytoskeleton, are poorly understood. Here, we investigate how spectrin re-organizes spatially and dynamically under the membrane during changes in cell mechanics. We find spectrin and acto-myosin to be spatially distinct but cooperating during mechanical challenges, such as cell adhesion and contraction, or compression, stretch and osmolarity fluctuations, creating a cohesive cortex supporting the plasma membrane. Actin territories control protrusions and contractile structures while spectrin territories concentrate in retractile zones and low-actin density/inter-contractile regions, acting as a fence that organize membrane trafficking events. We unveil here the existence of a dynamic interplay between acto-myosin and spectrin necessary to support a mesoscale organization of the lipid bilayer into spatially-confined cortical territories during cell mechanoresponse.

The excitable signal transduction networks: movers and shapers of eukaryotic cell migration

In response to a variety of external cues, eukaryotic cells display varied migratory modes to perform their physiological functions during development and in the adult. Aberrations in cell migration result in embryonic defects and cancer metastasis. The molecular components involved in cell migration are remarkably conserved between the social amoeba Dictyostelium and mammalian cells. This makes the amoeba an excellent model system for studies of eukaryotic cell migration. These migration-associated components can be grouped into three networks: input, signal transduction and cytoskeletal. In migrating cells, signal transduction events such as Ras or PI3K activity occur at the protrusion tips, referred to as 'front', whereas events such as dissociation of PTEN from these regions are referred to as 'back'. Asymmetric distribution of such front and back events is crucial for establishing polarity and guiding cell migration. The triggering of these signaling events displays properties of biochemical excitability including all-or-nothing responsiveness to suprathreshold stimuli, refractoriness, and wave propagation. These signal transduction waves originate from a point and propagate towards the edge of the cell, thereby driving cytoskeletal activity and cellular protrusions. Any change in the threshold for network activation alters the range of the propagating waves and the size of cellular protrusions which gives rise to various migratory modes in cells. Thus, this review highlights excitable signal transduction networks as key players for coordinating cytoskeletal activities to drive cell migration in all eukaryotes.

The Roles of Signaling in Cytoskeletal Changes, Random Movement, Direction-Sensing and Polarization of Eukaryotic Cells

Movement of cells and tissues is essential at various stages during the lifetime of an organism, including morphogenesis in early development, in the immune response to pathogens, and during wound-healing and tissue regeneration. Individual cells are able to move in a variety of microenvironments (MEs) (A glossary of the acronyms used herein is given at the end) by suitably adapting both their shape and how they transmit force to the ME, but how cells translate environmental signals into the forces that shape them and enable them to move is poorly understood. While many of the networks involved in signal detection, transduction and movement have been characterized, how intracellular signals control re-building of the cyctoskeleton to enable movement is not understood. In this review we discuss recent advances in our understanding of signal transduction networks related to direction-sensing and movement, and some of the problems that remain to be solved.

Long-Tailed Unconventional Class I Myosins in Health and Disease

Long-tailed unconventional class I myosin, Myosin 1E (MYO1E) and Myosin 1F (MYO1F) are motor proteins that use chemical energy from the hydrolysis of adenosine triphosphate (ATP) to produce mechanical work along the actin cytoskeleton. On the basis of their motor properties and structural features, myosins perform a variety of essential roles in physiological processes such as endocytosis, exocytosis, cell adhesion, and migration. The long tailed unconventional class I myosins are characterized by having a conserved motor head domain, which binds actin and hydrolyzes ATP, followed by a short neck with an isoleucine-glutamine (IQ) motif, which binds calmodulin and is sensitive to calcium, and a tail that contains a pleckstrin homology domain (PH), a tail homology 1 domain (TH1), wherein these domains allow membrane binding, a tail homology 2 domain (TH2), an ATP-insensitive actin-binding site domain, and a single Src homology 3 domain (SH3) susceptible to binding proline rich regions in other proteins. Therefore, these motor proteins are able to bind actin, plasma membrane, and other molecules (adaptor, kinases, membrane proteins) that contribute to their function, ranging from increasing membrane tension to molecular trafficking and cellular adhesion. MYO1E and MYO1F function in host self-defense, with a better defined role in innate immunity in cell migration and phagocytosis. Impairments of their function have been identified in patients suffering pathologies ranging from tumoral processes to kidney diseases. In this review, we summarize our current knowledge of specific features and functions of MYO1E and MYO1F in various tissues, as well as their involvement in disease.

Excitable networks controlling cell migration during development and disease

The directed movements of individual, groups, or sheets of cells at specific times in particular locations bring about form and complexity to developing organisms. Cells move by extending protrusions, such as macropinosomes, pseudopods, lamellipods, filopods, or blebs. Although many of the cytoskeletal components within these structures are known, less is known about the mechanisms that determine their location, number, and characteristics. Recent evidence suggests that control may be exerted by a signal transduction excitable network whose components and activities, including Ras, PI3K, TorC2, and phosphoinositides, self-organize on the plasma membrane and propagate in waves. The waves drive the various types of protrusions, which in turn, determine the modes of cell migration. Acute perturbations at specific points in the network produce abrupt shifts in protrusion type, including transitions from pseudopods to filopods or lamellipods. These observations have also contributed to a delineation of the signal transduction network, including candidate fast positive and delayed negative feedback loops. The network contains many oncogenes and tumor suppressors, and other molecules which have recently been implicated in developmental and metabolic abnormalities. Thus, the concept of signal transduction network excitability in cell migration can be used to understand disease states and morphological changes occurring in development.

Control of actin dynamics during cell motility

Actin polymerization is essential for cells to migrate, as well as for various cell biological processes such as cytokinesis and vesicle traffic. This brief review describes the mechanisms underlying its different roles and recent advances in our understanding. Actin usually requires "nuclei"-preformed actin filaments-to start polymerizing, but, once initiated, polymerization continues constitutively. The field therefore has a strong focus on nucleators, in particular the Arp2/3 complex and formins. These have different functions, are controlled by contrasting mechanisms, and generate alternate geometries of actin networks. The Arp2/3 complex functions only when activated by nucleation-promoting factors such as WASP, Scar/WAVE, WASH, and WHAMM and when binding to a pre-existing filament. Formins can be individually active but are usually autoinhibited. Each is controlled by different mechanisms and is involved in different biological roles. We also describe the processes leading to actin disassembly and their regulation and conclude with four questions whose answers are important for understanding actin dynamics but are currently unanswered.

A Multiparametric and High-Throughput Assay to Quantify the Influence of Target Size on Phagocytosis

Phagocytosis by macrophages represents a fundamental process essential for both immunity and tissue homeostasis. It consists in the uptake of pathogenic or cellular targets larger than 0.5 μm. For the biggest particles, the phagocytic process involves a massive reorganization of membrane and actin cytoskeleton as well as an important intracellular deformation all in a matter of minutes. The study of the role of the size of objects in their phagocytosis has led to contradictory results in the last decades. We designed a method using confocal microscopy, automated image analysis, and databases for fast quantitative analysis of phagocytosis assays. It yields comprehensive data on the cells and targets geometric and fluorescence intensity parameters, automatically discriminates internalized from external targets, and stores the relationship between a cell and the targets it has engulfed. We used two types of targets (solid polystyrene beads and liquid lipid droplets) to investigate the influence of size on the phagocytic uptake of macrophages. The method made it possible not only to perform phagocytic assays with functionalized droplets and beads of different sizes but to use polydisperse particles to further our understanding of the role of size in phagocytosis. The use of monodisperse and polydisperse objects shows that whereas smaller monodisperse objects are internalized in greater numbers, objects of different sizes presented simultaneously are internalized without preferred size. The total surface engulfed by the cell is thus the main factor limiting the uptake of particles, regardless of their nature or size. A meta-analysis of the literature reveals that this dependence in surface is consistently conserved throughout cell types, targets' nature, or activated receptors.

Membrane-cytoskeletal crosstalk mediated by myosin-I regulates adhesion turnover during phagocytosis

Phagocytosis of invading pathogens or cellular debris requires a dramatic change in cell shape driven by actin polymerization. For antibody-covered targets, phagocytosis is thought to proceed through the sequential engagement of Fc-receptors on the phagocyte with antibodies on the target surface, leading to the extension and closure of the phagocytic cup around the target. We find that two actin-dependent molecular motors, class 1 myosins myosin 1e and myosin 1f, are specifically localized to Fc-receptor adhesions and required for efficient phagocytosis of antibody-opsonized targets. Using primary macrophages lacking both myosin 1e and myosin 1f, we find that without the actin-membrane linkage mediated by these myosins, the organization of individual adhesions is compromised, leading to excessive actin polymerization, slower adhesion turnover, and deficient phagocytic internalization. This work identifies a role for class 1 myosins in coordinated adhesion turnover during phagocytosis and supports a mechanism involving membrane-cytoskeletal crosstalk for phagocytic cup closure.

Eukaryotic Cell Dynamics from Crawlers to Swimmers

Movement requires force transmission to the environment, and motile cells are robustly, though not elegantly, designed nanomachines that often can cope with a variety of environmental conditions by altering the mode of force transmission used. As with humans, the available modes range from momentary attachment to a substrate when crawling, to shape deformations when swimming, and at the cellular level this involves sensing the mechanical properties of the environment and altering the mode appropriately. While many types of cells can adapt their mode of movement to their microenvironment (ME), our understanding of how they detect, transduce and process information from the ME to determine the optimal mode is still rudimentary. The shape and integrity of a cell is determined by its cytoskeleton (CSK), and thus the shape changes that may be required to move involve controlled remodeling of the CSK. Motion in vivo is often in response to extracellular signals, which requires the ability to detect such signals and transduce them into the shape changes and force generation needed for movement. Thus the nanomachine is complex, and while much is known about individual components involved in movement, an integrated understanding of motility in even simple cells such as bacteria is not at hand. In this review we discuss recent advances in our understanding of cell motility and some of the problems remaining to be solved.

Do lipids shape the eukaryotic cell cycle?

Successful passage through the cell cycle presents a number of structural challenges to the cell. Inceptive studies carried out in the last five years have produced clear evidence of modulations in the lipid profile (sometimes referred to as the lipidome) of eukaryotes as a function of the cell cycle. This mounting body of evidence indicates that lipids play key roles in the structural transformations seen across the cycle. The accumulation of this evidence coincides with a revolution in our understanding of how lipid composition regulates a plethora of biological processes ranging from protein activity through to cellular signalling and membrane compartmentalisation. In this review, we discuss evidence from biological, chemical and physical studies of the lipid fraction across the cell cycle that demonstrate that lipids are well-developed cellular components at the heart of the biological machinery responsible for managing progress through the cell cycle. Furthermore, we discuss the mechanisms by which this careful control is exercised.

Fronts and waves of actin polymerization in a bistability-based mechanism of circular dorsal ruffles

During macropinocytosis, cells remodel their morphologies for the uptake of extracellular matter. This endocytotic mechanism relies on the collapse and closure of precursory structures, which are propagating actin-based, ring-shaped vertical undulations at the dorsal (top) cell membrane, a.k.a. circular dorsal ruffles (CDRs). As such, CDRs are essential to a range of vital and pathogenic processes alike. Here we show, based on both experimental data and theoretical analysis, that CDRs are propagating fronts of actin polymerization in a bistable system. The theory relies on a novel mass-conserving reaction–diffusion model, which associates the expansion and contraction of waves to distinct counter-propagating front solutions. Moreover, the model predicts that under a change in parameters (for example, biochemical conditions) CDRs may be pinned and fluctuate near the cell boundary or exhibit complex spiral wave dynamics due to a wave instability. We observe both phenomena also in our experiments indicating the conditions for which macropinocytosis is suppressed.

Circular dorsal ruffles (CDRs) are important for the vesicular uptake of extracellular matter, but the basis of their wave dynamics is not understood. Here, the authors propose and experimentally test a bistable reaction-diffusion system, which they show accounts for the typical CDR expansion and shrinkage and for aberrant formation of pinned waves and spirals.